(1) Field of the Invention
The present invention relates to a switching system, and more particularly, to a technique for a telecommunication conference in a digital switching system incorporated in a time division multiplex network, which technique enables a mixture of voice signals and permits simultaneous communication between three or more subscribers for conference purposes.
(2) Description of the Related Art
Generally, many subscribers are accommodated by a digital switching system. Conversations are usually performed between two subscribers in accordance with switching control. Recently, however, switching systems are being designed to enable telecommunication conferences wherein conversations are performed between three, four or more subscribers. A telecommunication conference is achieved relatively easily by a digital switching system operated in a time division multiplex network through a mixing process using circuit switching. With the above-mentioned mixing process, voice signals transmitted from a plurality of subscriber terminals (telephone sets) are mixed and then distributed to the subscribers concerned therewith.
In prior art switching systems, the mixing process is achieved using a mixing trunk (TRK). As a result, two problems arise. First, if the trunk is designed as a trunk for three persons, the trunk is necessarily limited to use by no more than three subscribers for a conference. Similarly, if it is designed as a trunk for four persons, the trunk is necessarily limited to use by no more than four subscribers. When a trunk is required to cope with more subscribers, the related hardware, such as the incoming-buffer registors and outgoing-buffer registers, necessarily becomes large in scale.
Second, a suitable number of the mixing trunks has to be always available so as to deel with traffic conditions in the switching system, which conditions are determined in advance when the switching system is built.